1. Field of the Invention
The present invention relates an open type magnetic resonance imaging (hereinafter will be referred to as MRI) apparatus using a super conducting magnet which does not give a pressing feeling to a person to be inspected and, more specifically, relates to such MRI apparatus in which maintenance and management of cooling medium for the super conducting magnet is accurate and easy.
2. Conventional Art
An MRI apparatus which produces a tomographic picture image of a human body by making use of Nuclear Magnetic Resonance (NMR) phenomenon is broadly used in medical facilities. In such MRI apparatus a magnet for generating a uniform static magnetic field in a space where a person to be inspected is laid is provided. As such static magnetic field generation magnet, a permanent magnet, a normal conducting magnet and a super conducting magnet are conventionally used. Among these, application of the super conducting magnet to the MRI apparatus is widespreading because with the super conducting magnet a strong static magnetic field can be generated, thereby, a high speed image taking and a high quality picture image can be realized.
As conventional coils for static magnetic field generation long and narrow cylindrical solenoid coils were generally used, however, an MRI apparatus having such a magnet structure which requires a person to be inspected to be laid in a long and narrow space gives the person to be inspected a pressing feeling and is not appropriate for inspecting such as a claustrophobia and a child. In contrast thereto, an open type MRI apparatus is in these days becoming popular in which a pair of magnets generating comparatively low magnetic field are disposed so as to locate the image taking space therebetween and the image taking space is opened in which the person to be inspected is laid. Further, for the magnet in such open type MRI apparatus, in place of the conventional permanent magnet and normal conducting magnet coils, a development of a magnet in which super conducting coils are assembled is now advancing.
However, in such open type MRI apparatus, since a cryostat which accommodates the super conducting coils is disposed after being divided into a plurality of units, a different maintenance and management from one for the conventional super conducting magnet is required.
Namely, almost all of the conventional super conductive magnets were constituted by winding wire around a cylindrical bobbin in a solenoid shape and were disposed in a single liquid helium vessel. Since direction of the generated magnetic field is in horizontal direction, the axis of the solenoid coils is also disposed in horizontal direction. For this reason, even if an amount of liquid helium in the vessel decreases by about 20% each part of respective turns of the super conducting coil is still immersed in the liquid helium in the vessel, thereby, the super conducting coil can be kept stably at temperature of 4.2xc2x0 K. Further, when managing the amount of liquid helium, it was sufficient if the single liquid level is measured.
Contrary thereto, in an open type MRI apparatus in which a pair of solenoid coils are disposed above and below the image taking space where the person to be inspected is laid so that the axis of the pair of solenoid coils directs in vertical direction, when the liquid helium in the vessel decreases a part of turns of the super conducting coil completely emerges from the liquid helium surface, thereby, the super conducting condition of the coil is suddenly broken which possibly prevents measurement of MRI.
Further, since the decreasing speeds of liquid helium in the upper and lower cryostats are not necessarily the same, the lower limit values of the liquid helium amount in the upper and lower cryostats which can keep super conductivity of the coils are different, it is necessary to manage the liquid helium amount for every upper and lower cryostats.
Namely, in the open structured super conducting magnet, in order to enhance magnetic field generation efficiency it is preferable to dispose respective super conducting coils near to the picture taking space as much as possible, therefore, the super conducting coil in the upper cryostat is disposed at the bottom portion thereof and the super conducting coil in the lower cryostat is disposed at the top portion thereof. As a result, even if the liquid helium in the upper and lower cryostats is evaporated likely, the coil in the lower cryostat emerges early from the liquid helium surface. For this reason, the management of liquid helium amount in the upper and lower cryostats is to be performed independently and separately which makes the management difficult of the open type super conducting magnet.
The above problem becomes further important for an open type super conducting magnet in which a plurality of coils of different configurations are disposed in each cryostat in order to improve magnetic field uniformity and to limit leakage magnetic field distribution space.
JP-A-11-16718 (1999) discloses a provision of individual level sensors for upper and lower cryostats for a super conducting magnet, however, nowhere discloses any specific separate liquid helium monitoring system using the individual level sensors.
The present invention has been achieved in view of the above problems and an object of the present invention is to provide an open type MRI apparatus using a super conducting magnet in which maintenance and management of cooling medium for the super conducting magnet can be performed accurately and easily, thereby, such as breaking of super conducting condition of the super conductive magnet can be surely prevented.
An MRI apparatus of the present invention which achieves the above object comprises: a static magnetic generating device including a pair of upper and lower static magnetic field generating sources which are disposed so as to locate a space for laying a person to be inspected therebetween and respective containers each accommodating one of the pair of upper and lower magnetic field generating sources, each of the magnetic field generation sources is constituted by not less than one super conducting coil, each of the containers contains cooling medium which keeps the super conducting coil at a predetermined temperature, and further comprises measurement means each of which is disposed in every containers and sends out an electrical signal representing an amount of cooling medium therein and computing means which receives the electrical signals from the respective measuring means and computes amount of cooling medium and/or variation in amount of cooling medium in the respective containers.
In the present invention, since the amount of cooling medium in every respective containers is measured, management of the amount of cooling medium becomes accurate and easy.
In the MRI apparatus according to the present invention further comprises a display unit which displays an amount of cooling medium and/or a variation in amount of cooling medium in respective containers together with allowable lower limit values for the amount of cooling medium in every respective containers. Through confirming the amount of cooling medium and/or variation in the amount of cooling medium in every respective containers which are displayed together with the allowable lower limit value therefor via the display unit, a decrease of cooling medium in the respective containers can be visually grasped, thus, even when the lower limit values of the cooling medium for the respective containers are different, the management thereof is facilitated. The amount of cooling medium and/or variation in the amount of the cooling medium can be displayed either by numeral values in digital or by graphs formed by plotting the amounts with respect to time.
Further, the MRI apparatus of the present invention, preferably, comprises means which generates an alarm either when an amount of cooling medium reaches the predetermined lower limit value or when a variation in the amount of cooling medium exceeds the predetermined value.
For the alarming conventional measures such as alarming lamp and alarm by sound such as beep sound can be used. Through such alarming even when cooling medium suddenly decreases by any causes, an immediate action therefor can be taken, and an accident of the cryostats due to man-made mistakes can be prevented.
Further, the MRI apparatus of the present invention comprises a control means which controls the measurement means and the computing means so that the measurement by the measurement means and computation by the computing means are performed with a desired time interval.
Through performing the measurement with a predetermined time interval set by the control means, an amount of cooling medium in the upper and lower containers can be monitored automatically and periodically.